A micropolarimeter is a scientific instrument used to measure the angle of rotation caused by passing polarized light through an optically active substance, such as certain liquid crystal materials. The micropolarimeters are usually utilized in conjunction with image sensors, such as digital cameras, to obtain images showing the polarization components measured by the micropolarimeters.
U.S. Pat. No. 5,327,285 to S. M. Faris discloses several methods of manufacturing micropolarizers, which includes selective bleaching/treating a polyvinyl alcohol (PVA) film with patterned photoresist on top, selective PVA film etching, including chemical etching, photochemical etching, eximer laser etching and reactive ion etching, with patterned photoresist on top, mechanical cutting/milling and electrically controlled liquid-crystal cell with patterned indium tin oxide (ITO) electrodes.
U.S. Pat. No. 7,385,669 B2 to Z. Wang, et al. disclose several methods of creating twisted nematic (TN) polymerizable liquid crystal micropolarizers including: (a) two-step UV exposure; (b) e-field alignment; (c) multi-rubbing and (d) photo-induced alignment. Two LC microdomains are formed, where one is in TN state and the other one is in isotropic state. In the TN state, LC molecules are twisted by 90° and it is optically equal to a polarization rotator; while in the isotropic state, the arrangement of LC molecules is random.
In the non-patent literature, “A polarization contrast retina that uses patterned iodine-doped PVA film,” Proceeding of 22nd European Solid-State Circuits Conference, pp. 308-311, 1996, Z. K. Kalayjian, et al. demonstrate a micropolarizer fabrication method by patterning iodine-doped PVA film. In there, masking and etching steps are used to undope iodine selectively in regions of PVA. The dichroic effect of said undoped regions is destroyed by removing the iodine from the polymer sheet.
In the non-patent literature, “Fabrication of thin-film micropolarizer arrays for visible imaging polarimetry,” Applied Optics, vol. 39, no. 10, pp. 1486-1492, 2000, J. Guo, et al. demonstrate a fabrication method of thin-film micropolarizer arrays. A dichroic dye material is first spin-coated then rubbed to form a thin polarizing film. Reactive ion etching is applied successively to pattern the thin polarizing film.
In the non-patent literature, “Liquid-crystal micropolarizer array for polarization-difference imaging,” Applied Optics, vol. 41, no. 7, pp. 1291-1296, 2002, C. K. Harnett, et al. present a liquid-crystal micropolarizer array with evaporated gold film as the orientation layers. Gold is evaporated to LC substrate with predetermined direction and liftoff is used to pattern the gold film. One more LC microdomain needs one more gold evaporation and liftoff. Two LC microdomains are formed with their gold evaporation directions perpendicular to each other.
In the non-patent literature, “An analog VLSI chip emulating polarization vision of octopus retina,” IEEE Transactions on Neural Networks, vol. 17, no. 1, pp. 222-232, 2006, M. Momeni, et al. demonstrate a micropolarizer array made of YVO4. An aluminum film is evaporated on top of YVO4 crystal then patterned by liftoff to form the birefringent micropolarizer array.
In the non-patent literature, “Fabrication of a dual-tier thin film micro polarization array,” Optics Express, vol. 15, no. 8, pp. 4994-5007, 2007, V. Gruev, et al. demonstrate a dual-tier micropolarizer array for extracting partial linear polarization information. This dual-tier micropolarizer array is formed by successively laminating and etching two PVA layers. The two PVA layers are laminated with a predetermined angle between their polarizing axis orientations.
Previously reported micropolarizers have been generally adequate for their intended applications. However, only linear polarization information is provided by these micropolarizers. Complete polarization information tends to be more complex and its real-time extraction requires simultaneous polarimetries of unpolarized, linearly polarized and circularly polarized components of incident light.
Another disadvantage is the large pixel size of existing micropolarimeters, where high resolution polarization imaging is not possible due to the pixel size mismatch between the micropolarimeter array and the image sensing array of advanced solid-state image sensor.
Furthermore, it is desired to reduce the complexity of the micropolarimeter array fabrication and its compatibility with solid-state image sensor fabrication. With previously reported selective-etching-based methods, the fabrication process of the micropolarimeter array for extracting complete polarization information is extremely complex. In addition, the fabrication compatibility between the micropolarimeter array and the solid-state image sensor is an important factor of the overall manufacturing cost.